The most widely used electrode couple in lithium-ion storage batteries is the LiCoO2/C (carbon) couple. However, the compound LiC6 that is formed when lithium ions are inserted into a carbon structure, for example, graphite, is explosion hazardous. The drawbacks of graphite include alteration of the structure during operation and an initial loss of capacity up to 20%. These drawbacks lead to the necessity of using other materials as the anode, for example, lithium-titanium spinel, Li4Ti5O12 (in alternate notation, Li4/3Ti5/3O4).
[J. Li, Y-L. Jin, X.-G. Zhang, H. Yang. Microwave solid-state synthesis of spinel Li4Ti5O12 nanocrystallites as anode material for lithium-ion batteries //Solid State Ionics.-2007,-V. 178.-P. 1590-1594.; Xu J., Wang Y., Li Z., Zhang W. F. Preparation and electrochemical properties of carbon-doped TiO2 nanotubes as an anode material for lithium-ion batteries //Journal of Power Sources.-2008,-V. 175.-P. 903-908; Li4Ti5O12 as anode in all-solid-state, plastic, lithium-ion batteries for low-power applications//P. P. Prosini, R. Mancini, L. Petrucci et al. //Solid State Ionics.-2001, V.-144.-P. 185-192.]
Anode materials for lithium-ion electrochemical cells, lithium-titanium spinels, in particular Li4Ti5O12, began to be regarded as promising material for lithium batteries since the mid-1980s.
[D. W. Murphy, R. J. Cava, S. Zahurak, A. Santoro. Ternary LixTiO2 phases from insertion reactions//Solid State Ionics.-1983.-V. 9-10.-P. 413-417; K. M. Colbow, J. R. Dahn and R. R. Haering. Structure and Electrochemistry of Spinel Oxides LiTi2O4 and Li4/3Ti5/3O4//J. Power Sources.-1989.-V. 26,-N. 3/4.-P. 397-402; T. Ohzuku, A. Ueda, N. Yamamoto. Zero-Strain Insertion Material of Li[Li1/3Ti1/3]O4 for Rechargeable Lithium Cells//J. Electrochem. Soc.-1995.-V. 142.-I. 5.-P. 1431-1435.]
One formula unit of this substance is capable of accepting three lithium ions. The electrochemical discharge process is described by the following reaction:Li4Ti5O12+3e−+3Li+=Li7Ti5O12 
The theoretical specific capacity of the substance in the process described by the equation is 175 mA·hour/g. As a result of this reaction, the initial structure of the substance of the spinel type shifts to a metastable phase with an ordered structure of the NaCl type. As distinct from the majority of the known electrode materials for lithium-ion ECCs, the electromotive force of which depends on the degree of discharge of the material (expressed as the subscript x of lithium in the formula of the active substance, for example, LixCoO2), in this case it is determined by the two-phase equilibrium, (Li4Ti5O12)spinel(Li7Ti5O12)quasi-NaCl, and therefore is constant and is E°=1.55 V with respect to metallic lithium.
The crystallographic notation of this concentration transition for the tripled formula unit of the classical spinel (3×AB2O4=A3B6O12, where A and B are atoms at the tetrahedral and octahedral sites of the structure, respectively) has the following appearance:LiA3(LiTi5)BO12+3Li→Li6(LiTi5)O12≡B′6B6O12 
This formula expresses the formation of two non-equivalent sublattices with octahedral sites of the 16c and 16d type. Since the initial tetrahedral sites in the process described become octahedral through filling with additional lithium atoms (8a→16c), this concentration phase transition is possible only if all tetrahedral sites in the initial spinel structure are occupied by mobile lithium. If an atom not capable of transition to 16c octahedral sites in the course of the process under discussion is present at 8a tetrahedral sites, the process is “frozen,” and a reversible charge-discharge electrochemical reaction, as occurs, for example, in the case of doping of a lithium-titanium spinel with iron, is impossible.
[P. Kubiak, A. Garcia, M. Womes, L. Aldon, J. Olivier-Fourcade, P.-E. Lippens, J.-C. Jumas. Phase transition in the spinel Li4Ti5O12 induced by lithium insertion. Influence of the substitutions Ti/V, Ti/Mn, Ti/Fe.//J. Power Sources.-2003.-V. 119-121.-P. 626-630].
As the electromotive force of the potential-generating reaction in relation to metallic lithium, E0Li, is equal to 1.55 V, thus with the use of this substance as an anode, the electromotive force of the cell is substantially lower than with the use of the traditional carbon electrode (C6-LiC6; E0Li≈0.1 V), and resulting specific energy of the material is not large, but this drawback is redeemed by the unique cycicability of the material. Since the volume changes are negligible, 0.07%, in the case of the Li4Ti5O12→Li7Ti5O12 transition, this is regarded as a highly favorable factor fostering cycle life, as the mechanical degradation of the macrostructure of the electrode is excluded. The low cost of the initial raw material, i.e. titanium compounds, is also of considerable significance for marketing prospects of the material. In addition, the charged form of the material, Li7Ti5O12, is entirely safe, unlike LiC6, and all the more so, metallic lithium, which are spontaneously inflammable in air (for example, when the electric cell is destroyed).
[E. Ferg, R. J. Gummow, A. de Kock, M. M. Thackeray. Spinel Anodes for Lithium-Ion Batteries//J. Electrochem. Soc.-1994.-V. 141.-I. 11.-P. L147-L150; K. Zaghib, M. Armand, M. Gauthier. Electrochemistry of Anodes in Solid-State Li-Ion Polymer Batteries//J. Electrochem. Soc.-1998.-M. 145.-I. 9.-P. 3135-3140; A. D. Robertson, L. Trevino, H. Tukamoto, J. T. S. Irvine. New inorganic spinel oxides for use as negative electrode materials in future lithium-ion batteries//J. Power Sources.-1999.-V. 81-82.-P. 352-357; see also RF patent No. 2304325, published 10 Aug. 2007].
The aggregate of these properties makes it possible to regard the lithium-titanium spinel as a highly promising anode material for lithium electric cells. However, the low electron conductivity of the material and, as a result, the low degree of operating capacity extraction (not more than 160 mA·hour/g, e.g., not more than 90% of the theoretical), especially at high current densities, is the main impediment to the use of this material in the mass production of electric cells.
The reason for the low electron conductivity is the absence of charge carriers in the structure of this substance, making it virtually a dielectric. The titanium in this structure has the highest oxidation state, 4+, and the charge balance can be expressed by the formula Li+4Ti4+5O2−12. Since the titanium 4+ ion has a 3d0 electronic configuration, the valence band is completely filled and the conductivity band is completely empty. The electron conductivity, therefore, as for other dielectrics, is determined by the “prehistory” of the specimen, in the first place by the presence and concentration of impurities that produce donor and acceptor levels, as well as by its intrinsic defects. According to the data of many publications, the electron conductivity of Li4Ti5O12 under normal conditions lies in the interval, 10−8-10−13 Ohm−1·cm−1.
[C. H. Chen, J. T. Vaughey, A. N. Jansen, D. W. Dees, A. J. Kahaian, T. Goacher, M. M. Thackeray. Studies of Mg-Substituted Li4-xMgxTi5O12Spinel Electrodes (0<x<1) for Lithium Batteries//J. Electrochem. Soc.,-2001,-V. 148,-P. A102.; I. A. Leonidov, O. N. Leonidova, O. F. Samigullina, M. V. Patrakeev. Structural Aspects of Lithium Transfer in Solid Electrolytes Li2xZn2−3xTi1+xO4 (0.33≦x≦0.67)//Zhurnal strukturnoy khimii [Journal of Structural Chemistry].-2004.-V. 45.-No. 2.-P. 262-268; M. Wilkening, R. Amade, W. Iwaniak, P. Heitjans. Ultraslow Li diffusion in spinel-type structured Li4Ti5O12-A comparison of results from solid state NMR and impedance spectroscopy//Phys. Chem. Chem. Phys.-2007-V. 9-P. 1239-1246; see also U.S. Pat. No. 7,211,350, date of issue 1 May 2007; U.S. Pat. No. 6,379,843, date of issue 30 Apr. 2002 (Europatent EP 0 845 825 B1, issued 21 Jan. 2004); U.S. patent applications 2007/0238023, published 11 Oct. 2007; U.S. 2007/0243467, published 18 Oct. 2007].
Several methods are known for increasing the electron conductivity of a lithium-titanium spinel. The creation of a two-phase composition comprising an electrochemically active substance, in this case Li4Ti5O12, and an electroconductive additive that is uniformly distributed among the lithium titanate particles, is possible. Both mechanical mixtures of the electrode material and various forms of carbon, as well as chemically deposited films of carbon on lithium titanate, are used. The development of this logic has led to the creation of a material with a carbon electroconductive coating on Li4Ti5O12 particles.
[L. Cheng, X. Li, H. Liu, H. Xiong, P. Zhang, Y. Xia. Carbon-Coated Li4Ti5O12 as a High Rate Electrode Material for Li-Ion Intercalation//J. Electrochem. Soc.-2007.-V. 154.-I. 7.-P. A692-A697; R. Dominko, M. Gaberscek, M. Bele, D. Mihailovic, J. Jamnik. Carbon nanocoatings on active materials for Li-ion batteries.//J. Eur. Cer. Soc.-2007.-V. 27.-I. 2-3.-P. 909-913; U.S. patent application 2008/0315161, published 25 Dec. 2008]
Metals, in particular copper and silver, as well as an intermetallic compounds, titanium nitride (TiN), have also been proposed as such electroconductive additives. The current characteristics of the anode material are improved by means of the creation of disperse two-phase compositions, obtained in particular by in situ methods. The use of compositions with electroconductive polymers has also been proposed.
[S. Huang, Z. Wen, J. Zhang, X. Yang. Improving the electrochemical performance of Li4Ti5O12/Ag composite by an electroless deposition method//Electrochimica Acta.-2007.-V. 52.-I. 11.-P. 3704-3708; S. Huang, Z. Wen, B. Lin, J. Han, X. Xu. The high-rate performance of the Newly Designed Li4Ti5O12/Cu composite anode for lithium ion batteries//Journal of Alloys and Compounds.-2008.-V. 457,-I. 1-2.-P. 400-403; S. Huang, Z. Wen, J. Zhang, Z. Gu, Xi. Li4Ti5O12/Ag composite as electrode materials for lithium-ion battery//Solid State Ionics.-2006. V. 177.-I. 9-10.-P. 851-855; M. Q. Snyder, S. A. Trebukhova, B. Ravdel, M. C. Wheeler, J. DiCarlo, C. P. Tripp, W. J. DeSisto. Synthesis and characterization of atomic layer deposited titanium nitride thin films on lithium titanate spinel powder as a lithium-ion battery anode//J. Power Sources-2007-V. 165.-P. 379-385; H. Yu, H. Xie, A. F. Jalbout, X. Yan, X. Pan, R. Wang, High-rate characteristics of novel anode Li4Ti5O12/polyacene materials for Li-ion secondary batteries//Electrochimica Acta.-2007.-V. 53.-I. 12,-P. 4200-4204.]
While increasing the total electrical conductivity of the composition, these methods do little to improve the local conditions of discharge of the particles of the electrode material beyond the limits of the direct electrode material-electroconductive additive contact. Also, conductive surface additives operate only at the initial stage of the electrochemical intercalation of lithium into the material. It should be taken into consideration that the Li7Ti5O12 phase obtained from the initial material upon its discharge is a very good conductor, since it contains a high concentration of Ti3+ (3d1), i.e., electrons in the conduction band (in accordance with the formula Li7Ti3+3Ti4+2O12). Therefore, the Li4Ti5O12 particles (starting from the composition Li4+δTi5O12, where δ≈0.1, i.e. ˜ 1/30 of the total capacitity) are already coated with a current-conducting layer of Li7Ti5O12 at the initial stages of the process, which makes the presence of a preliminarily created layer superfluous. Subsequently the process takes place along the interphase boundary between Li4Ti5O12 and Li7Ti5O12, i.e., the lithium ions and electrons enter the reaction zone through the Li7Ti5O12 layer. The reverse process (discharge) must proceed in an entirely different manner, since a non-conductive layer of Li4Ti5O12 is formed in its turn at the surface of the Li7Ti5O12 particles (at least at high rates of discharge) and the lithium ions and electrons now enter the reaction zone through the Li4Ti5O12 layer. In this case the presence of an electroconductive layer at the surface of the particles is not crucial for some significant improvement of the kinetics of the electrode process.
[S. Scharner, W. Wepner, P. Schmid-Beurmann. Evidence of Two-Phase Formation upon Lithium insertion into the Li1.33Ti1.67O4 Spinel//J. Electrochem. Soc.-1999.-V. 146.-I. 5.-P. 857-861; W. Lu, I. Belharouak, J. Liu, K. Amine. Electrochemical and Thermal Investigation of Li4/3Ti5/3O4Spinel//J. Electrochem. Society,-2007.-V.154.-P. A114-A118; F. Ronci, P. Reale, B. Scrosati, S. Panero, V. R. Albertini, P. Perfetti, M. di Michiel, J. M. Merino. High-Resolution In-Situ Structural Measurements of the Li4/3Ti5/3O4 “Zero-Strain” Insertion Material.//J. Phys. Chem. B,-2002.-V. 106,-P. 3082].
A second approach to improving electron conductivity is the introduction of impurities, or doping, i.e., the partial substitution of structure-forming ions by other ions, as a rule, in a different charge state. In the process the concentration of the doping element is such that the type of crystalline structure of the initial substance is unchanged. Impurity charge defects serving as the carriers of electric current are formed in the substance as a result of such substitution. In case of the absence or extremely low concentration of its intrinsic current carriers, the occurrence of such extrinsic carriers may alter the conductivity of the substance by many orders of magnitude.
In the known methods of producing anode materials, substantial substitution of titanium by molybdenum with simultaneous reduction of molybdenum to the Mo4+ state results in improvement of the conductivity of the final product. Thus, the measured electrical conductivities for Li4Ti4.5Mo0.5O12, Li4Ti4MoO12, and Li4Ti3.5Mo1.5O12 are 1.6, 2.8, and 5.8 10−3 Ohm−1·cm−1, respectively. However, molybdenum is not a “good” doping element for the mass production of lithium-titanium spinel, as the cost of its reagents is high.
[Z. Zhong. Synthesis of Mo4+ Substituted Spinel Li4Ti5-xMoxO12//Electrochem. Solid-State Lett.,-2007,-V. 10-N. 12-P. A267-A269; U.S. patent application 2009/0004563, published 1 Jan. 2009].
In other known methods, the substitution of titanium in Li4Ti5O12 by iron, nickel, vanadium, and manganese results in the appearance of these elements both at the octahedral and the tetrahedral sites of the structure. This depresses the concentration phase transition and degrades the electrochemical properties of the material. The results of electrochemical studies of these substituted compositions leads to the same conclusions. Attempts have been made at the substitution of titanium by other elements as well, for example, Al, Ga, and Co, or at substitution in an anion sublattice.
[S. Scharner, W. Wepner, P. Schmid-Beurmann. Evidence of Two-Phase Formation upon Lithium insertion into the Li1.33Ti1.67O4 Spinel//J. Electrochem. Soc.-1999.-V. 146.-I. 5.-P. 857-861; A. D. Robertson, L. Trevino, H. Tukamoto, J. T. S. Irvine. New inorganic spinel oxides for use as negative electrode materials in future lithium-ion batteries//J. of Power Sources.-1999.-V. 81-82.-P. 352-357; A. D. Roberston, H. Tukamoto, J. T. S. Irvine.//J. Electrochem. Soc. 146 (1999) P. 3958; P. Kubiak, A. Garcia, M. Womes, L. Aldon, J. Olivier-Fourcade, P.-E. Lippens, J.-C. Jumas. Phase transition in the spinel Li4Ti5O12 induced by lithium insertion. Influence of the substitutions Ti/V, Ti/Mn, Ti/Fe.//J. Power Sources.-2003.-V.119-121.-P. 626-630; S. Huang, Z. Wen, X. Zhu, Z. Lin. Effects of dopant on the electrochemical performance of Li4Ti5O12 as electrode material for lithium ion batteries.//J. Power Sources.-2007.-V. 165.-I. 1.-P. 408-412; S. Huang, Z. Wen, Z. Gu, X. Zhu. Preparation and cycling performance of Al3+ and F− co-substituted compounds Li4AlxTi5-xFyO12-y.//Electrochimica Acta-2005.-V. 50.-I. 20.-P. 4057-4062].
A new approach was demonstrated in a method according to which, by means of heat treatment of samples of lithium titanate at high temperatures in a reducing gaseous environment, materials of the composition, Li4Ti5O12-δ, where δ≦0.012, that are reduced in the anion sublattice, are obtained. Such nonstoichiometry for oxygen leads to a sharp increase in electronic conductivity, approximately to 10−6-3·10−6 S/cm under normal conditions (temperature 298 K) due to the partial removal of oxygen with preservation of the crystalline structure. However, exceeding the limiting value δ=0.012 leads to loss of stability of the crystalline structure and decay of the spinel phase into compounds with structures of the Li2Ti3O7 and γ-Li2TiO3 types.
[V. Gorchkov, O. Volkov. Lithium titanate and method of forming the same. U.S. Pat. No. 7,541,016, issued 2 Jun. 2009].
The traditional approach of intensification of the electrode process in active materials of CPSs—the production of an electrode with a high specific surface area and small effective particle size (for example, by the sol-gel or dry-spray methods), does not lead to improvement of the discharge characteristics, since the resulting increase in the surface of the electrolyte and electrode contact does not decrease electric resistance, and the extremely low electronic conductivity of the electrode is a limiting factor of the process. Decreasing the size of the particles leads to deterioration of cycleability at high discharge rates, especially at low temperatures.
[J. L. Allen, T. R. Jow, J. Wolfenstine. Low temperature performance of nanophase Li4Ti5O12.//J. of Power Sources.-2006.-V. 159.-I. 2. P. 1340-1345; U.S. Pat. No. 7,368,097, date of issue 6 May 2008; U.S. Pat. No. 6,881,393, date of issue 19, April 2005].
All known anode materials based on lithium-titanium spinel for CPSs produced by the methods described above possess inadequate electronic conductivity, and as a consequence thereof, limited electrochemical capacity and cycleability at high operating currents that impede use in devices with high power consumption. At the same time, the cost of the initial components for such technologies is high and the production methods are complicated.
There is a known anode material for lithium-ion CPSs based on lithium-titanium spinel of the chemical formula, Li4Ti5O12-x, where x is the deviation from stoichiometry for oxygen, within the limits of 0<x<0.02, that is closest to the invention being applied for in technical essence and result to be achieved.
The known method—the prototype of the production of said anode material for lithium-ion CPSs, includes preparation of a mixture of the initial components, that contain lithium and titanium, by means of homogenization and pulverization, with subsequent heat treatment of the prepared mixture in a controlled atmosphere of inert and reducing gases. Wherewith hydrogen, hydrocarbons, and carbon monoxide are used as reducing gases and the reduction process (heat treatment) is run at a temperature of 450° C. for 30 minutes.
[see US application No. US 2007/0238023, IPC H01M 4/48, C01G 23/04, published 11 Oct. 2007; patent issued to V. Gorshkov, O. Volkov. Lithium titanate and method of forming the same. U.S. Pat. No. 7,541,016 B2. Jun. 2, 2009].
The characteristics of the known anode material for lithium-ion CPSs produced by method described above are given in the Table.
Although the known material possesses high electrochemical capacity, and good cycleability, it does not operate at high discharge currents due to low electronic conductivity.